Most computers include a disk drive which is used for data storage. The disk drive includes a rotatable disk having a magnetic coating on at least one side thereof. A read/write head is disposed adjacent the surface, and an actuator can move the read/write head approximately radially with respect to the surface, so that the head can write data to the surface and/or read data from the surface. The surface on the disk is conceptually divided into a plurality of concentric data tracks, which can each store data.
Early disk drives included a read/write head having a single read/write element, which was used both for writing data and reading data. However, there has been a progressively increasing demand for disk drives with significantly higher storage densities, and one result is that new types of heads have come into common use, examples of which include the magneto-resistive (MR) head, and the giant magneto-resistive (GMR) head. These MR and GMR heads typically have one element for writing data and a separate element for reading data, and these read and write elements are physically spaced from each other.
As is known in the art, a head can be positioned with respect to a disk by using feedback control based on servo information read from the disk with a read element of the head. In a head with spaced read and write elements, the read element is used to position the head relative to the disk not only for reading, but also for writing. One aspect of this is that, as the head is moved relative to the disk, the orientation of the read and write elements varies with respect to the tracks on the disk, such that the write element is typically aligned with a track that is different from the track with which the read element is aligned. Consequently, in order to correctly position the write element over a selected track for the purpose of writing data to that track, the read element must be positioned at a location which is radially offset from the selected track. This radial offset is referred to as a “microjog”, and has a magnitude which varies as the head moves radially with respect to the disk. Techniques have been developed for calculating microjog values, and have been generally adequate for their intended purposes, but they have not been satisfactory in all respects.
As one aspect of this, there are existing disk drives in which the disk is rotatably supported in a removable cartridge, and in which the head is movably supported in a drive unit that can removably receive the cartridge. A given drive unit must be able to work with any of several similar and interchangeable cartridges, and any given cartridge must be capable of working in any of a number of compatible drive units. The removability of the cartridge introduces a number of real-world considerations into the system, and these considerations affect the accurate calculation of a microjog value.
For example, the cartridges have manufacturing tolerances which vary from cartridge. Thus, from cartridge to cartridge, there will be some variation relative to the cartridge housing of the exact position of the axis of rotation of the disk. As another example, two different cartridges may have slightly different mechanical seatings when they are inserted into the same drive unit. In fact, a given cartridge may experience different mechanical seatings on two successive insertions into the same drive unit. Real-world variations of this type cause small variations in the orientation of the read/write head with respect to the tracks on the disk, and thus affect accurate calculation of a microjog value.
One of the major components of head position error is called repeatable runout (RRO). RRO that occurs at the disk rotating frequency may be called 1F runout. There are several possible causes for 1F runout, such as an unbalanced spindle, or a non-ideal spindle bearing.
In order to realize higher data storage densities in systems of the type which utilize removable cartridges, it is desirable to be able to use read/write heads that facilitate high storage densities, especially read/write heads that have spaced read and write elements, such as MR and GMR heads. What is needed is a system that compensates for any changes in the microjog that may occur.
Further, if a removable cartridge is dropped, the disk may slip within the clamp, resulting in large RRO. As the head moves back and forth in order for the read element to follow the RRO, the write element, which is spaced some distance away from the read element, does not remain centered over the intended write position. If the track density is high enough, the microjog error caused by the RRO will increase, eventually resulting in degraded performance in reading the data. In addition, if the RRO changes once the disk has been written, a subsequent write may cause encroachment. What is needed is a system that can compensate for microjog error caused by the RRO.